Proteolysis of von Willebrand factor (vWF) normally occurs through the action of a plasma enzyme that has recently been characterized; it accounts for the small quantities of cleavage products normally present in the circulation, and its inhibition can lead to the disease called thrombotic thrombocytopenic purpura (TTP). We have developed a rapid assay to evaluate the cleavage of vWF and have characterized patients with a TTP-like syndrome to detect those with low vWF cleaving protease activity. The assay does not require specialized reagents and can be completed within 6-8 hours on patient plasma. We have studied a group of 50 masked plasmapheresis samples in collaboration with hematology investigators form UNC medical school, and a paper is in press presenting the results of this study. We have begun a collaboration with Dr. William Stetler-Stevenson (NCI) to attempt to express the protein in mammalian cells.